Mature black alder shows intermittent events of high stem methane emissions

Summary Tree stems can be a source of the greenhouse gas methane (CH 4 ). However, assessments of the global importance are complicated by a lack of research and a high variability between ecosystems. Here, we determined the contribution of emissions from tree stems of mature black alder ( Alnus glutinosa (L.) Gaertn.) to overall CH 4 exchange in two temperate peatlands. We measured emissions from stems and soils using closed chambers in a drained and a wet alder forest over two years. Further, we studied the importance of alder leaves as substrate for methanogenesis in an incubation experiment. Stem CH 4 emissions were shortlived and occurred only during times of inundation at the wet site. The drained site did not show stem emissions and the soil acted as a small CH 4 sink. The contribution of stem emissions to the overall CH 4 budget was below 0.3% in both sites. Our results show that also mature black alders intermittently can be a source of CH 4 . However, the low share of stem-mediated CH 4 emissions in both forests may indicate that this pathway is only of minor relative importance in temperate peatlands.